Light emitting diodes (LEDs) have become a desirable replacement for traditional lighting methods, including incandescent, fluorescent and halogen lighting. Compared to these types of lights, LEDs are much more energy efficient and may have much longer product lifetimes. However, the materials used to make LEDs typically limit the colors possible in an LED lighting application.
Semiconductor nanocrystals are typically tiny crystals of II-VI, III-V, IV-VI, or I-III-VI materials that have a diameter between 1 nanometer (nm) and 20 nm. In the strong confinement limit, the physical diameter of the nanocrystal is smaller than the bulk excitation Bohr radius causing quantum confinement effects to predominate. In this regime, the nanocrystal is a 0-dimensional system that has both quantized density and energy of electronic states where the actual energy and energy differences between electronic states are a function of both the nanocrystal composition and physical size. Larger nanocrystals have more closely spaced energy states and smaller nanocrystals have the reverse. Because interaction of light and matter is determined by the density and energy of electronic states, many of the optical and electric properties of nanocrystals can be tuned or altered simply by changing the nanocrystal geometry (i.e. physical size).
Single nanocrystals or monodisperse populations of nanocrystals exhibit unique optical properties that are size tunable. Both the onset of absorption and the photoluminescent wavelength are a function of nanocrystal size and composition. The nanocrystals will absorb all wavelengths shorter than the absorption onset, however, photoluminescence will always occur at the absorption onset. The bandwidth of the photoluminescent spectra is due to both homogeneous and inhomogeneous broadening mechanisms. Homogeneous mechanisms include temperature dependent Doppler broadening and broadening due to the Heisenberg uncertainty principle, while inhomogeneous broadening is due to the size distribution of the nanocrystals. The narrower the size distribution of the nanocrystals is, the narrower the full-width at half max (FWHM) of the resultant photoluminescent spectra will be. In 1991, Brus wrote a paper reviewing the theoretical and experimental research conducted on colloidally grown semiconductor nanocrystals, such as cadmium selenide (CdSe) in particular. (Brus L., Quantum Crystallites and Nonlinear Optics, Applied Physics A, 53 (1991)). That research, precipitated in the early 1980's by the likes of Efros, Ekimov, and Brus himself, greatly accelerated by the end of the 1980's as demonstrated by the increase in the number of papers concerning colloidally grown semiconductor nanocrystals in past years.
Although nanocrystal based LEDs have been developed and commercialized to date by Evident Technologies, the relative lifetime of these LEDs has generally been limited to less than 1000 hrs. Relative LED lifetime is typically measured by comparing the total power output after a burn in period to the time required to achieve a total power output at 50% of the original signal. This shorter lifetime is primarily due to the sensitivity of the semiconductor nanocrystals to water, oxygen, light, and heat. As a result it is desired to increase the LED lifetime in order to enable high margin business and more general lighting applications.